Platyhelminthes is a phylum of dorsoventrally flattened acoelomate but triploblastic animals having bilateral symmetry and blind sac body plan.
Due to their flattened nature, Platyhelminthes are also called flatworms.
The term Platyhelminthes was coined by Gegenbaur (1859).
Characteristics of Phylum Platyhelminthes:
Flatworms are free-living commensal or parasitic organisms. The free-living worms may be terrestrial or freshwater but a majority of endoparasitic.
The body is dorsoventrally flattened, leaf-like or ribbon-like.
The parasitic worms bear suckers and hooks to get attached themselves with hosts.
Platyhelminthes are triploblastic animals with three germ layers- ectoderm, endoderm and mesoderm.
Spaces between the body organs are filled with a mesodermal connective tissue, called parenchyma.
Flatworms exhibit a definite organ system level of organisation. Flatworms are the first and the simplest animals that have organs and organ systems. The systems are simple.
Excretion and osmoregulation occur by peculiar cells called flame cells or protonephridia or solenocytes. These are ammonotelic.
The alimentary canal is branched or unbranched with an opening called a mouth.
The nervous system includes a pair of an anterior ganglion (brain ganglion) and a ventral nerve cord.
A skeletal system is absent. Flatworms are, therefore, soft. In certain cases scleroprotein cuticle is present.
Flatworms have definite anterior and posterior ends. The concentration of sense organs at the anterior end marks the beginning of cephalization.
Most of the flatworms are hermaphrodites but can reproduce asexually and by sexual methods. In sexually reproducing organisms, cross-fertilization occurs in most cases.
Fertilization is internal and development is indirect through many larval stages.
The power of regeneration is well marked in some flat worms.
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